Within the semiconductor industry, lithographic printers with reticles (also called masks) having device patterns have been used to pattern photoresist layers for several years. A reticle typically is a glass plate with chrome elements on the plate used to define a pattern. The chrome elements are opaque to the radiation source used in the lithographic printer. The radiation source generates a sinusoidal wave having a specific wavelength. A discussion of lithographic printing and the diffraction limitations of lithographic printing appears on pages 274-276 of VLSI Technology edited by S.M. Sze (.COPYRGT.1983), which is herein incorporated by reference and hereinafter referred to as Sze. Resolution is a measure of how small of an image that can be patterned with a given set of optical parameters. Depth of focus is a measure of the vertical distance over which an image can be printed. Formulas for calculating resolution and depth of focus appear on page 276 of Sze.
Ideally, a very small resolution and a depth of focus at least as deep as the thickness of the photoresist layer are desired. As can be seen by the equations in Sze, the resolution and depth of focus are both affected by changing the wavelength of the radiation used or the numerical aperture of the lens within the lithographic printer. Therefore, smaller resolution can be achieved by lowering the wavelength or increasing the numerical aperture, but the smaller resolution is typically at the expense of the depth of focus.
In recent years, semiconductor devices have been characterized by the miniaturization of circuits and the spaces between the circuits used to form the devices. When a dimension of the pattern to be printed during lithography is smaller than the resolution, the pattern formed in the photoresist layer is distorted by diffraction. The prior art problem of diffraction limitations is disclosed in Japanese Patent Application Number 63-295350 by Okamoto, which is herein incorporated by reference and hereinafter referred to as Okamoto. FIG. 18 of Okamoto illustrates the light intensity when the dimensions of a chrome element and spaces on opposite sides of the chrome element are smaller than the resolution. The light intensity at the surface of the wafer is distorted from the square wave pattern achieved immediately after the light has passed through the reticle.
A prior art attempt to print a small dimensional pattern is to use chrome elements in conjunction with phase-shifting elements. FIG. 19 of Okamoto teaches a common example chrome used with phase shifters. A phase-shifting element is placed over alternate spaces between chrome elements. The phase-shifting element shifts the phase of the light about 180.degree. out of phase compared to the adjacent space. The radiation from the space with the phase-shifting element destructively interferes with the light from the space without the phase-shifting element. The light intensity at the surface of the wafer is closer to the desired intensity pattern than the light intensity of FIG. 18. The reticle used in FIG. 19 uses both an opaque material and a phase-shifting material, which makes manufacture the reticle more complicated and requires additional manufacturing steps.